The Arizona ACLU said Tuesday that police in that state have used racial profiling to target Hispanics under that state's strict immigration laws, filing a legal complaint against the city of South Tucson and opening up another legal battle against the state's "show your papers" law.

In a legal complaint filed with South Tucson, the American Civil Liberties Union's Arizona chapter said city police illegally detained 23-year-old Alejandro Valenzuela, who the lawsuit said is in the country illegally but who has applied for tentative status to stay in the United States under President Obama's nondeportation policies.

The ACLU said a South Tucson police officer, whom it identified as Officer Paul South, singled out Mr. Valenzuela because he is Hispanic and said he was going to get him deported. The complaint says Officer South took Mr. Valenzuela to a Border Patrol station where he was held for five hours before being released, once the agents were satisfied he was eligible for Mr. Obama's nondeportation program.

"We've been informed of dozens of incidents where police have violated individuals' rights because of the 'show me your papers' law," said James Lyall, the ACLU of Arizona's Tucson-based border litigation attorney. "Officers are routinely harassing people who have committed no crime by demanding identification. Without major improvements to police policies and practices, these rights violations will persist."

South Tucson's attorney said she hadn't had time to fully review the complaint Tuesday. Messages seeking comment from the police chief and the police department spokesman weren't returned.

The ACLU's challenge comes even as immigration reform appears to be stalling as an issue in Washington, despite strong support from Mr. Obama.

A high-ranking House Republican told activists last week that there isn't enough time left this year to act on immigration bills, according to an Associated Press report. That leaves Congress with the choice of tackling immigration during an election year, which could be tricky, or putting it off until 2015.

Faced with those long odds, activists have vowed to redouble their efforts. On Tuesday, some of them announced a hunger fast to try to spur Congress to act this year.

Advocates have tried everything else, including civil disobedience and even arrest, and some of them said it's time to try prayer. They've set up a tent on the Mall in Washington to host their fasting sessions.

"It is being blocked, and the political commentators will tell you it would be a miracle — that's what they say — a miracle for immigration reform to pass. Sometimes you can organize and struggle and march as hard as you can, and sometimes all you can do is pray for a miracle," said the Rev. Jim Wallis, president of Sojourners, an evangelical group pushing for legalization.

Adding to the pressure, young illegal immigrants plan to descend on Washington next week to personally lobby members of Congress to approve legalization.

Activists are also demanding meetings with their elected representatives.

"What will ultimately get immigration reform over the finish line is outside pressure, not insider conventional wisdom," said Frank Sharry, executive director of America's Voice. "We are growing stronger as a movement every day and will continue to ratchet up pressure until we achieve reform."

For now, though, with reform stalled in Washington, attention will turn back to the states and the patchwork of immigration laws that have appeared.

Some states have approved in-state tuition rates or granting driver's licenses to illegal immigrants. Last month, California enacted a law preventing police from holding illegal immigrants for deportation if they have been accused of only minor crimes.

On the other side is Arizona, whose strict crackdown law passed in 2010, has been used as a model by a number of other states.

The Arizona law created state penalties associated with illegal immigration, and ordered state and local police to check the legal status of those they encounter in their normal duties whom they suspect of being in the country illegally.

The Supreme Court struck down the state penalties for illegal immigration, arguing that they intruded on the federal government's prerogatives. But the justices left intact the "show your papers" part of the law, saying as long as the checks were conducted quickly, they didn't violate anyone's rights. They also said the federal government could refuse to respond to the calls from local law enforcement.

The court did leave open the possibility of revisiting the law if police abused it, and the ACLU says that's what happened with Mr. Valenzuela.

They also presented a list of four other cases where they said local police used the law's powers to target Hispanics, including one where police stopped a car for having a burned-out taillight and ended up taking a passenger to federal deportation authorities, and another instance in which a U.S. citizen was jailed after he picked a water bottle out of the trash at a convenience store.

In the case of Mr. Valenzuela, the ACLU said he provided police with three forms of identification: his bus ID, his community college ID and an identification from a local worker center.

The ACLU accused Officer South of false imprisonment and asked for a $100,000 settlement as well as attorney fees.